1. Field of the Invention
The present invention relates to imaging apparatuses such as laser beam printers. In particular, the present invention relates to a fixing operation of such an imaging apparatus.
2. Description of the Related Art
Hitherto, in a thermal image-fixing device which is included in an imaging apparatus using a recording method such as an electrophotographic method or an electrostatic recording method, a so-called heat-roller-type heat-fuser has been widely used. In such a device, a toner image is fixed to a sheet as a permanent image when the sheet, which carries the toner image, travels through a pressure-nip region formed between a fusion roller and a pressure roller which rotate while being pressed to each other.
A heat-fuser in which electrical consumption is minimized by suspending electric supply during a standby mode is also known. The heat-fuser uses a film-heat method for thermally fixing toner images on sheets by transferring the sheets between a pressure roller and a heating element (a heater) mounted to a supporting unit via a thin film (a fusing film).
The two types of heat-fusers described above, which form thermal image-fixing devices, each include at least one fusion-temperature-measuring member for use in controlling temperature at which images are thermally fixed. The fusion-temperature-measuring member generally uses a thermistor. The fusion-temperature-measuring member is disposed in contact with the heater or the pressure roller and thereby measures the temperature of the heater or the pressure roller. Recently, a method for measuring the temperature for thermal fusion by measuring radiant heat from the heater or the pressure roller has also been studied.
FIGS. 5A, 5B, 5C, and 5D show the relationship of positions between a ceramic heater 11a serving as a heater element, thermistors 11d and 11e, and a sheet 501 or 502. The sheets 501 and 502 are transferred along arrows shown in the drawings. The dimension perpendicular to the transfer direction of each sheet is hereinafter referred to as the width. In FIG. 5A, a sheet having a maximum width that can be transferred in the image forming apparatus is sandwiched between the ceramic heater 11a and the pressure roller. In FIG. 5B, a sheet having a minimum width that can be transferred in the image forming apparatus is sandwiched by the ceramic heater 11a and the pressure roller. The ceramic heater 11a is fixed to a stay 11b which positions the ceramic heater 11a, as shown in FIG. 6.
The ceramic heater 11a opposes a pressure roller 10 with a fusing film 11c therebetween. When the pressure roller 10 is driven, a sheet which is sandwiched at a position (hereinafter referred to as a nip) at which the pressure roller 10 and the fusing film 11c are in contact with each other is conveyed. During this operation, a developer carried on the sheet is fixed onto the sheet as a result of the pressure from the pressure roller 10 and the heat supplied by the ceramic heater 11a. The main thermistor 11d is disposed in a position where a sheet passes, regardless of size. That is, the main thermistor 11d is disposed in a given position in a region 510 shown in FIG. 5B. The sub-thermistor 11e is disposed in a position where a part of a sheet having a maximum width which can be conveyed passes by and which a part of a sheet having a minimum width which can be conveyed does not pass by in a normal state. That is, the sub-thermistor 11e is disposed in a given position in region 511 shown in FIG. 5B.
FIG. 5C shows an example of temperature distribution in the ceramic heater 11a when the sheet having a minimum width is transferred. During the transfer of the sheet having a minimum width, gaps between the ceramic heater 11a and the pressure roller 10 are produced immediately outside the sheet because of the thickness of the sheet, and regions in which heat generated by the ceramic heater 11a is not transferred either to the sheet or to the pressure roller 10 are thereby produced. Therefore, the temperature in regions A and B of the ceramic heater 11a is increased as shown in FIG. 5C, when the temperature is set to a target temperature by using the main thermistor 11d. 
The temperature is particularly rapidly increased when a substance having a small width and a large thickness, for example, an envelope is conveyed. For example, when the target temperature is set to 200xc2x0 C., the temperature in the regions A and B is likely to increase to 300xc2x0 C.
When the temperature in end portions of the ceramic heater 11a is thus increased, the temperature is likely to exceed upper limits of the resistance to heat of the pressure roller 10 and the stay (the heater-supporting member) 11b. Therefore, there is a risk of damaging these components. When a plain paper having a large width is printed and the temperature in the end portions of the ceramic heater 11a is increased, there is a risk of a hot offset or the like due to an excessively raised temperature.
In order to prevent the temperature of the end portions from increasing when a sheet having a small width is transferred, the throughput is reduced when the temperature measured by the sub-thermistor 11e is increased, thereby suppressing the temperature rise in the end portions where no sheet portion passes. The sub-thermistor 11e is provided in addition to the main thermistor 11d in a position which no sheet portions pass by.
The temperature in the end portions increases at every fusion. Therefore, when the throughput is reduced, the temperature rise in the end portions decreases per unit time, and the components are thereby protected from being damaged.
However, if a user does not correctly set a side-restricting tray when setting envelopes or the like onto a multi-sheet-supply tray, the following problem will occur.
An imaging apparatus, in which a recording sheet having a width smaller than that of a sheet having a maximum width capable of being used in the apparatus is set in a widthwise-intermediate part of a sheet-supply tray (hereinafter referred to as a center-sheet-supply system), uses a side-restricting tray having two side-restricting members provided on the sheet-supply tray when envelopes or the like are supplied. The two side-restricting members are disposed symmetrical with respect to an intermediate part of the side-restricting tray and are movable in conjunction with each other. When a sheet is restricted at both sides thereof, the sheet is positioned at the widthwise-intermediate part (see FIG. 5B).
However, when envelopes or the like are offset toward one side of the sheet-supply tray as a result of the two side-restricting members being away from each other farther than the width of the envelopes or the like, the envelopes or the like are transferred along the regions at which the main thermistor 11d and the sub-thermistor 11e are provided, as shown in FIG. 5D, and no difference in temperature between the main thermistor 11d and the sub-thermistor 11e is detected. Consequently, the throughput will not be reduced; as a result, the temperature at positions where no parts of the envelopes or the like pass will increase, and there is a risk of damage to the components such as the pressure roller 10 and the stay 11b because the temperature is likely to exceed upper limits of the resistance to heat of these components.
When printing of a plain paper having a larger width is performed when the temperature in the end portions is increased, there is a risk of a hot offset or the like due to an excessively raised temperature.
Accordingly, it is an object of the present invention to provide an image forming apparatus and a method of operating the image forming apparatus, in which a fuser is protected from being damaged even when a user incorrectly sets a side-restricting tray and sheets, such as envelopes, having a small width and a large thickness.
To these ends, according to an aspect of the present invention, an imaging apparatus comprises a film-heat-type fuser over which a sheet with an image passes for fixing; a first temperature-measuring device for measuring temperature, disposed at a position on the fuser over which part of a sheet regardless of the size of the sheet will pass; a second temperature-measuring device for measuring temperature, disposed at a position on the fuser over which a part of a sheet having a maximum size will pass and over which a sheet having a smaller size will not pass when the sheet of smaller size is conveyed correctly; a temperature-control member for controlling the temperature of the fuser in accordance with the temperature measured by the first temperature-measuring device; a sheet-size-detector for determining the size of the sheet; and a throughput-control for controlling the throughput of the imaging apparatus. The throughput-control changes the throughput when the sheet-size-detector determines that the sheet has the smaller size and when the difference between the temperature measured by the first temperature-measuring device and the temperature measured by the second temperature-measuring device is less than or equal to a predetermined value. The measurement is performed as the sheet passes over the fuser for fixing.
The throughput-control may reduce the throughput when the sheet-size-detector determines that the sheet is the smaller size and when the difference between an average value of the temperature measured in a predetermined period by the first temperature-measuring device and another average value of the temperature measured in the predetermined period by the second temperature-measuring device is less than or equal to a predetermined value, the measurement being performed as the sheet passes over the fuser for fixing.
An imaging apparatus according to an aspect of the invention, wherein the throughput-control reduces the throughput when the sheet-size-detector determines that the sheet is the smaller size and when the difference between an average value of the temperature measured in a predetermined period by the first temperature-measuring device and another average value of the temperature measured in the predetermined period by the second temperature-measuring device is less than or equal to a predetermined value, the measurement being performed as the sheet passes over the fuser for fixing.
The throughput-control may suspend heating and transfer of the sheet when the sheet-size-detector determines that the sheet is the smaller size and when the difference between the temperature measured by the first temperature-measuring device and the temperature measured by the second temperature-measuring device is less than or equal to a predetermined value, the measurement being performed as the sheet passes over the fuser for fixing.
The throughput may be changed when the sheet-size-detector determines that the sheet is the smaller size and when the difference between the temperature measured by the first temperature-measuring device and the temperature measured by the second temperature-measuring device is less than or equal to a predetermined value for a predetermined number of sheets, the measurement being performed as each sheet passes over the fuser for fixing.
According to another aspect of the present invention, a method for operating an imaging apparatus provided with a film-heat-type fuser comprises the steps of determining whether a transferred sheet is smaller than a predetermined value; determining whether a side-restricting tray is set incorrectly for the sheet having a size smaller than the predetermined value; and changing throughput of the imaging apparatus when it is determined that the sheet has a size smaller than the predetermined value and when it is determined that the side-restricting tray is set incorrectly for the sheet having a size smaller than the predetermined value.
The smaller size of the sheet may comprise the size of an envelope.
The side-restricting tray may regulate the position of the sheet to be set thereon with respect to a widthwise-intermediate part of the side-restricting tray.
According to the present invention, an image forming apparatus, in which a fuser is protected from being damaged even when a user incorrectly sets a side-restricting tray and sheets, such as envelopes, having a small width and a large thickness, is obtainable.
Further objects, features and advantages of the present invention will become apparent from the following description of the preferred embodiments with reference to the attached drawings.